The present disclosure relates to a discharge lamp able to be operated at less than full rated power exhibiting excellent lumen maintenance and high luminous efficacy without suffering undesirable color shift. It finds particular application in connection with ceramic metal halide lamps having low thallium iodide and optionally low indium iodide in the dose thereof, and will be described with particular reference thereto.
High Intensity Discharge (HID) lamps are high-efficiency lamps that can generate large amounts of light from a relatively small source. These lamps are widely used in many applications, including retail display lighting, highway and road lighting, lighting of large venues such as sports stadiums, floodlighting of industrial and commercial buildings and shops, and projectors, to name but a few. The term “HID lamp” is used to denote different kinds of lamps. These include mercury vapor lamps, metal halide lamps, and sodium lamps. Metal halide lamps, in particular, are widely used in areas that require a high level of brightness at relatively low cost. HID lamps differ from other lamps because their functioning environment requires operation at high temperature and high pressure over a prolonged period of time. Also, due to their usage and cost, it is desirable that these HID lamps have relatively long useful lives and produce a consistent level of brightness and color of light. Although in principle HID lamps can operate with either an alternating current (AC) supply or a direct-current (DC) supply, in practice, the lamps are usually driven via an AC supply.
Discharge lamps produce light by ionizing a vapor fill material, such as a mixture of rare gases, metal halides and mercury, with an electric arc passing between two electrodes. The electrodes and the fill materials are sealed within a translucent or transparent discharge vessel that maintains the pressure of the energized fill materials and allows the emitted light to pass through it. The fill materials, also known as the lamp “dose,” emit a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, e.g. color temperatures, color renderings, and luminous efficacies.
Given current awareness in society surrounding the use of energy in a more efficient and economical manner, there is an increasing interest in the lighting industry to provide lamps capable of operation with reduced energy consumption, optimally without sacrificing lamp performance and particularly without undergoing an undesirable color shift. One solution would be to operate lamps at a reduced power level. The potential savings in energy consumption for commercial lighting purposes, as well as the opportunity to reduce consumption of our energy resources as a society, are substantial.
At least one drawback exists, however, in operating ceramic metal halide (CMH) lamp lighting at less than its full power rating. As the operating lamp power level is reduced, the color of emitted light shifts from white to green, correlating to an increase in the correlated color temperature (CCT) of the lamp by as much as 1000° K or more. CMH lamp color is primarily decided by the halide dose composition in the vapor phase in the arc tube. A typical CMH lamp, for example, contains NaI, TlI, CaI2 and one or more rare earth iodides, such as DyI3, HoI3, TmI3. When the CMH lamp is dimmed, the halide vapor pressure in the arc tube will drop with the reduction of arc tube temperature. However, the TlI vapor pressure drops more slowly than that of the rare earth halides. Because the TlI emits green light, and remains at a relatively high vapor pressure as compared to the other halides in the fill, the light emitted by the lamp exhibits a color shift from white to green at dimmed conditions. Such a shift in light color may have a considerable impact on commercial usage. For example, retail and display venues, which often employ CMH lamps due to their long life and focused light emissions, can suffer considerably from lighting that does not present items being displayed to their best advantage, i.e., under white light. The same is true for public venues where lighting contributes to the atmosphere or ambiance experienced by customers.
With current technology, lamp chemistries provide very beneficial properties on most performance metrics. However, when lamps are operated at reduced power to reduce energy consumption, these performance metrics may be altered, and specifically the color of the emitted light may be negatively affected, i.e. a color shift may occur. Attempts have been made to reduce the undesirable color shift that occurs when operating a lamp at less than 100% of its power rating by altering dose chemistry, but often these attempts if successful at reducing color shift have often resulted in other lamp metrics suffering. In other words, there is generally a trade-off in another performance parameter when the dose is changed to optimize a desired lamp characteristic. For example, in some instances where desirable emission color was retained, the lamp suffered from reduced efficacy and/or poor lumen maintenance over the life of the lamp. These parameters relate directly to the color of light emitted by the lamp, and therefore directly affect the satisfaction of the consumer using the lamp. Therefore, efforts aimed at solving emission color problems by changing the lamp dose have resulted in losses, and sometimes substantial losses, with regard to other performance and photometric parameters, even when the change in dose chemistry has been minimal. In most instances efforts to improve lamp color have done so at the expense of other important lamp parameters.
For example, U.S. Pat. No. 6,501,220, U.S. Pat. No. 6,717,364 and U.S. Pat. No. 7,012,375, disclose a TlI-free lamp dose that includes DyI3, TmI3 or HoI3, which are known to interrupt the tungsten halogen cycle in the CMH lamps. As a result, these lamps have reduced lumen maintenance. In addition, some of the above patents contain MgI2, which may prove beneficial with regard to dimming characteristics including no or substantially no color shift, but also causes reductions in lamp efficacy and lumen maintenance. So far, there is lacking a CMH lamp dose that can provide excellent dimming characteristics and at the same time provide good lumen maintenance and efficacy. The foregoing drawbacks have been a limiting factor to the widespread use of CMH lamps under dimming, energy saving conditions.
There exists, therefore, a need for a lighting solution that can be operated at less than nominal power, i.e. under dimming, in a more energy efficient manner without suffering a loss of the perceived white color of the emitted light, particularly without causing a shift toward a more green hue of emitted light, without reducing lumen maintenance, and without detracting from lamp efficacy. What is desired is a lamp capable of operating, at the consumer's choice, at a reduced power rating, up to as much as 50% less power, while maintaining a white light emission, good lumen maintenance and efficacy of the lamp.
Unexpectedly, the present invention achieves all of the foregoing desirable parameters, while causing no or only negligible losses in other performance and photometric parameters of the lamp. This is accomplished by employing a lamp dose including thallium iodide in an amount less than 1 mol % and, optionally, indium iodide also as less than 1 mol %, based on the entire halide till, with an optimization of other halides compositions. The result is a lamp exhibiting excellent performance with regard to lumens, efficacy, and exhibiting no perceived color shift.